US secretary of state warns that Lebanon's security is being threatened by Hezbollah's influence in regional conflicts.

US Secretary of State Rex Tillerson has warned that Hezbollah's influence in regional conflicts is threatening Lebanon's security.

Tillerson made the comments on Thursday after meeting with Lebanese President Michel Aoun and Prime Minister Saad Hariri on Thursday during a short stopover in Lebanon's capital, Beirut.

Hariri is part of a unity government under Aoun that also includes rival political factions such as those supported by Hezbollah, a popular Shia group which is represented in the Lebanese parliament and has a powerful armed wing.

The Iran-allied Hezbollah has been fighting in Syria alongside regional allies to defeat both anti-government rebels and al-Qaeda-linked fighters.

At a press conference alongside Hariri, Tillerson reiterated that the US does not recognise a distinction between the Hezbollah's political and military wings.

"It is impossible to talk about stability, sovereignty and security in Lebanon without addressing Hezbollah," Tillerson told reporters in Beirut.

"The US has considered Hezbollah a terrorist organisation for more than two decades now ... It is unacceptable for a militia like Hezbollah to operate outside the authority of the Lebanese government. The only legitimate defender of the Lebanese state is the Lebanese armed forces."

Border dispute

Tillerson also said that Washington was engaging with both Lebanon and Israel to ensure their border remained calm.

Tensions between the two countries have risen in recent months because of Israeli plans to build a wall at the border, and because of Lebanon's decision to begin exploring for oil and gas in an offshore block.

For his part, Hariri said that Lebanon has the right to "explore, exploit and develop" its natural resources in its territorial waters.

"What is ours is ours and what is Israel's is Israel's. We are trying to find solutions that will be fair to us and fair to everyone," Hariri said at the press conference.

"We want to move to a state of permanent cease fire but Israel's daily violations of our sovereignty hinder that process, as does Israel's escalating rhetoric. This needs to stop," Hariri said.

In his meeting with Tillerson earlier on Thursday, Aoun rejected Israeli claims over the two countries' dispute and called on the US to play an "effective role" in resolving it.

The US top diplomat's visit to Beirut was part of a five-country tour of the Middle East.

Later on Thursday he arrived in Turkey, where he is scheduled to hold talks with President Recep Tayyip Erdogan and Foreign Minister Mevlut Cavusoglu.